International agreement over Syria appears still more remote after an acrimonious G20 summit exposed the depth of divisions over military action.
David Cameron warned the world could not "contract out" its morality to a Russian veto at the United Nations Security Council over its response to the use of chemical weapons.
He claimed those blaming the regime for the poison attack which killed hundreds in a Damascus suburb and backing a strike against Bashar Assad's regime had "far the better of the argument".
But as Barack Obama returned to the US to continue efforts to win over a sceptical Congress, Vladimir Putin insisted most of the leading economic powers gathered in St Petersburg were opposed.
Any attack without a UN resolution would "violate the law", the Russian president declared, indicating that he was ready to give further military assistance to the Damascus regime if it was attacked. "Will we help Syria? We will. And we are already helping - we send arms, we co-operate in the economic sphere," he said. The staunch Assad ally continues to blame the chemical attack in Ghouta on anti-regime "militants" hoping for support from the outside world, something Mr Cameron said was "impossible to believe". In a calculated jab at the Prime Minister, Mr Putin listed those backing military action and added: "Mr Cameron is also in favour, but in Britain Parliament was against that."
Mr Obama was left relatively isolated at the G20, as only France indicated it was ready to join the US in an armed response, while Britain, Turkey, Canada and Saudi Arabia voiced support for robust action. The US president insisted there was "a growing recognition that we cannot sit idly by" and announced plans to make an address on Syria from the White House on Tuesday.
Mr Cameron voiced frustration at the argument heard around the table during talks on Syria that no action should be taken unless sanctioned by the UN Security Council. "If we accept that the only way a response can be made to a country that was massacring half or more than half of its people is if the UN Security Council votes positively, we are contracting out our foreign policy, our morality, to the potential of a Russian veto," he said. He added: "Many of us believe that there is a case for taking action when you are trying to prevent a humanitarian emergency. It's better with the UN Security Council resolution, but you cannot rule out taking action if you cannot get it. I think we should learn from some of the genocides we've seen in our world that there is an imperative for a line to be drawn." He did not attempt to deny Mr Putin's claim that the G20 divided by more than 50/50 in opposition to military action but said an "extremely powerful" case was made by those in favour.
The formal communique from the summit made no mention of Syria - which was not on the official agenda and was discussed over dinner - but 11 member states including the UK issued a joint statement supporting "efforts undertaken by the United States and other countries to reinforce the prohibition on the use of chemical weapons". It said the world "cannot wait for endless failed processes that can only lead to increased suffering in Syria and regional instability".
This week's summit has witnessed renewed strains on the UK-Russia relationship, with Mr Cameron issuing an impassioned rebuttal of the reported description by a Putin aide of Britain as a "small island" whose voice did not carry weight. In a lavishly patriotic riposte, the Prime Minister said the UK had not only helped abolish slavery and settle the Second World War, but "invented most of the things worth inventing". "Our music delights and amuses millions: The Beatles, Elgar, and slightly less congruously, One Direction have conquered the world. I'm thinking of setting this to music," he added - sparking a plethora of internet videos as well as comparisons with a scene from hit film Love Actually, in which Hugh Grant's prime minister interrupts the US president to say: "We may be a small country but we're a great one, too." Mr Putin's chief spokesman denied making the remark, but Downing Street demanded "clarification".
Mr Cameron also welcomed offers of new humanitarian assistance for Syria from Canada, Italy and Qatar, following his announcement of an additional £52 million in British aid. He said he wanted to "set the drumbeat" of momentum towards filling a three billion US dollar (£1.9 billion) shortfall in UN funds to assist millions of people affected by the two-and-a-half year civil war.